American President Cos. announced Monday first quarter earnings of $20.9 million on revenues of $525.6 million, up from $19.5 million on revenues of $460.1 million in the first quarter of 1987.

A major factor in the improved performance of the Oakland, Calif.-based transportation company was the sale of a 472-acre parcel of property near Sacramento, Calif. As a result, real estate operations contributed pretax income of $12.6 million on revenues of $14.6 million.Transportation operations generated pretax income of $24.1 million on

revenues of $500.3 million, down from pretax income of $31.8 million on

revenues of $444.7 million in last year's first quarter.

The company attributed the decline in pretax income from transportation operations to lower utilization of vessel capacity on the inbound routes from Asia and higher operating costs in Asia, both of which reflect the decline in the U.S. dollar.

It also cited a slight decline in average revenue per container and additional investments for the expansion of services in West Asia and China and for further development of the company's telecommunications and information resource capabilities.

APC, one of the biggest containership operators in the busy trans-Pacific shipping lanes as well as an increasingly active participant in domestic transportation, said the higher transportation revenues stemmed from increased international and domestic container volumes.

Export volumes from the United States to Asia were up 35 percent, while import volumes from Asia to the United States increased 6 percent and intra- Asia volumes increased 11 percent compared with the same quarter last year, the company said.

U.S. domestic transportation volumes also increased 24 percent over the first quarter of 1987, as the company continued to expand its double-stack train network.

Pretax income was $36.7 million in the first quarter, compared with $35.5 million in the first quarter of 1987.

Earnings a share came to 85 cents, fully diluted, up from 79 cents, in the same period last year.

The first quarter results were up sharply from the $14.2 million that APC earned in the fourth quarter of 1987 on revenues of $518 million.

Burton M. Strauss Jr., an analyst with Shearson Lehman Hutton, said the results contained no surprises. He is predicting that APC earnings for the year will come to $3.30 a share.

Graeme Anne Lidgerwood, a transportation analyst with First Boston Inc., said the results were a little better than she had expected, even factoring in the benefits from improved real estate sales.

The thing that struck me was that export volumes were up 35 percent, she said. I had figured 20 percent to 25 percent. That would be a very good showing. The ships are pretty full westbound.

Ms. Lidgerwood noted, however, that there is pressure on APC's margins, reflected in the fact that operating profits from transportation activities were down 9 percent despite a 13 percent gain in revenues. That apparently reflects lower rates on imports from Asia.